
Vj2^ 





TfiANSWE STEEITH OF LARGE SPEOCE BEAMS, 



FROM TESTS MADE AT THE MASSACHUSETTS INSTITUTE 



OF TECHNOLOGY. 



_A.:^r ADDRESS 



DELIVERED BEFOEE THE SOCIETY OP AETS OP THE MASSACHUSETTS 



INSTITUTE OF TECHNOLOaY. 



15^^ mOF. OAET^>.0 1L.JL1SZA. 



Copyrighted by Professor Gaetano Lakza. .^ 



BOSTON: 
Press of the Bostox Journal of Commerce Publishing Company. 



TEMSVERSE STUIGTE OF LARGE SPRDCE BEAKS. 



PROM TESTS MADE AT THE MASSACHUSETTS INSTITUTE 

OF TECHNOLOGY. 



DELIVERED BEEORE THE SOCIETY OP ARTS OP THE MASSACHUSETTS 






INSTITUTE OF TEOHNOLOaT. 



BY mOF. OJl^ET^lVO ILrA^lVZA.. 



From the Boston Journal of Commerce, June 24, 1882. 



For a long time, I have been very anxious, 
to supplement my instruction in the strength 
of materials with actual tests, so that the 
students, by performing the experiments 
themselves, might better understand the be- 
havior of the materials when under stress and 
strain. In order to accomplish this result, 
one or more testing machines were needed, 
and of sufficient power to make tests on such 
a scale as to furnish reliable information. 

In the summer of 1881 an opportunity pre- 
sented itself to purchase a testing machine of 
50,000 pounds capacity for a price which en- 
couraged me to hope for success, and, having 
found some kind friends who were willing to 
furnish the money, I proceeded to purchase 
the machine for the institute. It was adapted 
to exert a tensile stress on specimens of about 
12 inches in length, and also for the com- 
pression of small cubes. It was an old style 
Fairbanks machine, and was sold to me by 
Messrs. Eiehle Bros., of Philadelphia, who had 
just put in its place, in the shops of the 
Pennsylvania railroad, one of their own make 
of 100,000 pounds capacity. 

The machine when received consisted, as is 
shown in the first cut, of a compound scale 
beam hung from a cast iron frame, by means 



of which the tension is measured, and below 
this a lever for exerting the pull, operated at 
the farther end by a screw, as shown in the 
cut. This screw, which was only 12 inches 
long, I replaced by one which was 24 inches 
long. With the machine, as thus equipped, 
we have been able to perform quite a number 
of experiments on tension. Besides this, by 
altering the position of the lower lever and of 
the screw, we have been able to test speci- 
mens about three feet long. I have also 
devised a pair of clamps, which, with the aid 
of a micrometer screw, enables us to meas- 
ure the stretch of the specimen under any 
given load to ipithin one ten-thousandth of 
an inch. 

The following are the tensile experiments 
that have been made by the fourth year 
students of applied mechanics : 

1. Tensile strength and contraction of area 
of boiler plate, by Messrs. Snow and Manning. 

2. Strength of upset and of plain iron bolts, 
by Messrs. Johnson and Morrison. 

3. Tensile strength and contraction of area 
of steel boiler plate, by Messrs. Boss and 
Cheney. 

4. Tensile strength of wire rope, by MessiP, 
French and White. 



. ,L3 





i 


1 


l^^^l 


^^s^mmm^ 


lllllB^^fl t ^'^ 


^^^H 


fjM] ^^ ' J^^^^K' i^^^Jiii" 


■|ji|ffii 




||i||pll'ffc 



b. Streiigtli of riveted joints, by Messrs. 
Carson and Eipley. 

6. Modulus of elasticity of tire steel, by 
Messrs. Faunce and Munroe. 

These students have all given me written 
reports of their work, in the course of which 
many points of very considerable interest have 
been developed. Besides this, opportunity 
has been furnished the students. to. familiaxize.. 
themselves somewhat with the behavior of the 
metals under stress and strain by actual ex- 
periment. Nevertheless, it is not my object, 
this evening, to take up your time with a 
detailed account of these experiments on ten- 
sion, and I shall therefore pass at once to a 
consideration of the transformation that 
has been made in the machine to adapt it to 
the testing of large beams transversely loaded, 
and of the results obtained by using the 
machine as thus equipped, inasmuch as it has 
already revealed, in this department, some 
facts which, as I believe, are of great import- 
ance to builders, and which tend to overthrow 
certain misconceptions in regard to the trans- 
verse strength of spruce timber. Indeed, in 
my anxiety to have a testing machine in my 
department, the desire to test, by its means, 
full siae beams, has perhaps been uppermost in 
my thoughts, : for the data most commonly 
given us for thie computation of wooden beams 
in our text books and engineers' hand books 
have been derived from the results of experi- 
ments on beanas of a very much smaller size 
than those used in practice, and hence upon 
pieces not containing the knots and defects 
that are almost invariably found in larger 
pieces. 

The transformation referred to is shown 
in the second Cut. The lower lever shown 
in the first cut was removed, and the holder 
of the compound lever, was connected by 
means of a steel rod and turn-buckle, with 
one end of a lever of equal arms placed below, 
this lever having a 12-inch leverage and being 
connected at its other end by means of a chain, 
with the yoke shown in the second cut. Two 
hard pine beams, each 20 inches deep, 10 
inches wide and 26 feet long, were placed 
across the timbers of the machine in such a 
way that the chain already referred to should 
be midway between them. Two common 
jackscrews, each in a pair of wrought iron 
stirrups, are placed at a distance apart, depend- 



ing upon the span of the beam to be tested, 
the latter being placed, as shown in the second 
cut, upon the jackscrews and under the yoke. 
The jackscrews are then screwed up, and the 
beam to be tested is thus raised at its two 
ends, and hence loaded at the point where the 
yoke is attached. In the cut, the beam repre- 
sented is a 4x12 inch spruce beam, the jack- 
screws being placed respectively at distances 
of 4.5 and 11.5 feet from the point where the 
yoke is attached. The fracture is shown in 
the figure. 

As to the capabilities of the machine, any 
beam can be broken by its means, which is 
not more than 25 feet long (between supports) 
and which does not require more than a load 
of 50,000 pounds to break it. As to its accu- 
racy, I will say that I have the written certifi- 
cate of Messrs. Fairbanks, Brown & Co. for 
the correctness of both the compound scale 
beam and also the equal armed lever, the' first 
of which they put in order, and the second 
they examined and found correct, it having 
been made at the Norway Iron Works. Be- 
sides this, however, the machine as a whole 
has been tested and proved correct by means 
of a steelyard of 1450 pounds capacity (for 
whose correctness I have also a written cer- 
tificate from Messrs. Fairbanks, Brown & Co.), 
by suspending the steelyard from a rod sup- 
ported on the jackscrews and attaching to it 
the chain of the equal armed lever. 

As to the tests that have been made thus 
far, they have been but few, for the reason 
that the machine has only been adapted to 
the testing of transverse stress since March 1, 
1882. Indeed, in the early part of the session, 
while the students were making their tensile 
tests, I, and my assistant, Mr. Wilkes, were 
making the calculations and drawings neces- 
sary for the transformation to a transverse 
machine ; and subsequently the diflerent parts 
were made for me of steel, at cost, through 
the kindness of Messrs. Naylor & Co. All this 
necessarily required time, and it was only in 
February that the equal armed lever was 
completed, and then the remainder of the 
month was required for its erection, and the 
preparation of the accessory apparatus. 

The following table gives the list of the 
tests that have been made by the students, 
and immediately after I will give a detailed 
account of them : 



TABIiE OF TESTS 

MADE BY STUDENTS OP THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



si 

so 0) 





a 


a 




.-. 


V 




sn 


<u 




Qi 


& 








s 










^ 


o 


a 


B 




cS m 




<v 






^ 






a 


2§ 


p 


1 


2 xl2 


6' 8 1' 




4 xl2 


6' & If 


3 


2 X12 


15' 


4 


2x9 


6' 7.5'' 


5 


2 xl2 


15' 


5(a) 


2 xl2 


7' 


6 


2.75X 9 


6' 8 " 


7 


3x9 


4' 


8 


3x9 


10' 


9 


3 X 9 


15' 


10 


3%xl2 


20' 


11 


2.0 X13.5 


10' 


12 


3.75x12 


16' 


13 


3.75x12 


16' 


14 


7 x2 


7' 


15 


1.7.')X 6.75 


7' 


16 


3x9 


6' 8 " 


17 


3x9 


6' 8 " 


18 


3.9 xl2 


16' 












ti'— ' > O} tB 



111° 



Me*5vs. Ely, Heins 
and Snelling. 



Messrs. Foss and 
Stebbins. 



Messrs. Johnson 
and Morrison 



Framed at ends. 

Header. 

Load at middle. 



r Load 4.5' from one 
I support. 
Ditto moi'tised. 
Load at middle. 



("Load distrib. at 4 
1 points, 16" apart. 
f Load 4.5' from one 
I support. 



6,504 
10,338 
5,894 
7,322 
5,586 
8,982 
7,586 
11,086 
6,086 
5,086 
6,586 
9,585 
7,585 

9,085 
1,944 

4,785 
9,985 

16,744 
12,585 



5,526 
5,389 
5,237 



307 
299 
291 



4,082 
3,285 
4,.508 
5,651 
4,253 
3,787 
3,271 



183 
2.50 
314 
237 
210 



8,748 
7,562 



486 
420 



4,961 
5,218 



13 )60.099 
4.623 



921 

898 
873 



548 
751 



709 
631 

645 



822 
827 



1,468 
1.260 




All the beams tested by the students have 
beeu of spruce, for the reason that it seemed 
better to keep to one kind of timber until we 
should have a sufficient number of results to 
justify us in drawing some conclusions, and 
also because spruce is very extensively used 
in construction. The attempt was made to 



secure such ordinary stock as is commonly 
used in building. The lumber for the tests 
1, 2, 3, and 4, was secured through the carpen- 
ter of the institute, at the place where he 
usually buys his lumber, and he was requested 
to obtain the same quality of stock as he 
ordinarily buys. The greater part of 



6 



remaining lumber was obtained from Messrs. 
Stetson & Moseley, and all that they furnished 
is stated to be merchantable stock, or such 
stock as they use to fill their orders. I ought 
to except, however, Nos. 14 and 15, which 
had been seasoning on the wharf since 1877, 
and were quite small. Some of the later sticks 
were bought from Mr. J. W. Leatherbee. 

The first test was made on a 2x12 inch beam, 
framed at each end into a 4x12 inch beam, 
with the framing joint shown in the figure. 
The beam was loaded at the middle, and gave 
way by splitting at the point shown in the 
figure. 

The second test was made on a header, 4x12 
inches and 6 feet 8 inches long, framed into 
the two trimmers, with a double tenon and a 
joint bolt, as shown in the sketch; the load 
was equally divided between the four points 
directly above the mortises cut for the tail 



off rapidly — all night, the next morning the 
beam bore only 3650 pounds before breaking. 

No. 4 calls for no special comment. 

No. 5 had the heart of the wood not at mid- 
dle of the depth, and after it had been broken 
a portion of it, cut from one side of the mid- 
dle, was tested again with a span of seven 
feet. This piece is marked 5 (a) in the table. 
It will be seen that it bore only 8982 
pounds before breaking, the reason of this 
being that long before it broke the cracks 
at the heart had so opened that a portion 
of the beam about two inches in depth 
was not aiding in any way the bearing of the 
load, as it was simply connected by shreds, 
and could be easily shaken with the hand. 

Nos. 5, 6, 7, 8, 9, 10 and 11 call for no special 
comment other than their age, which I shall 
refer to later. 

No. 12 contained quite a number of knots 




l)eatus, which mortises were filled up with the 
tenoijs themselves. The first breakage, which 
occurred under a load of about 9000 pounds, 
consisted of a splitting along the middle of 
the depth, and the final breakage of a splitting 
along the lower line shown in the sketch. The 
remainder of the tests were all made on 



No. 3 was a 2x12 inch beam, and while the 
load reached momentarily 5894 pounds, which 
I have called the breaking load, the beam had 
opened a crack at about the middle of its 
depth, extending for about one-half its length, 
at a much smaller load, and the load (5894 
pounds) having been left on — though falling 



and defects, and was in appearance as well as 
in fact quite a poor stick ; it was tested with 
one concentrated load as described in the table, 
and gave a modulus of rupture of only 3271 
pounds per square inch. 

No. 13 was a very good stick, and it was 
tested with mortises cut for a double tenon 
just below the place where the load was ap- 
plied ; of course no modulus of rupture can be 
estimated but if its breaking weight be com- 
pared with that of No. 18, which was of about 
the same quality, some idea can be formed of 
the loss of strength due to the mortising. 

In regard to the age and place of felling of 
the timber, I have the following information 



from Messrs. Stetson & Moseley : Nos. 5, 6, 7, 
8, 9, 10, 11, 12, 13, 16, 17, and 18 were cut at 
Bangor, Me.; Nos. 14 and 15 at Jerusalem 
township, Franklin Co., Me. Nos. 5, 9, 10, 12, 
13, 16, 17, and 18 were cut in the season of 
1881; Nos. 6, 7, 8, and 11, season of 1880; Nos. 
14 and 15, winter of 1877. 

As to the quality of the timber, it was, as I 
have said, with the exception of Nos. 14 and 
15, all common merchantable stock ; some of 
it, that cut in 1880, was rather drier than is 
ordinarily used ; some of the sticks were bet- 
ter than others, as will readily be seen by 
comparing the moduli. No. 18 was selected 
by Mr. Wilkes at Mr. Leatherbee's, and was 
the best stick of that size in the pile at the 
time; it was considerably better than the 
average, having no large knots' and very slight 
cracks, and being very straight grained. The 
average of the thirteen specimens tested gives 
a modulus of rupture of 4623 pounds per 
square inch, the lowest being 3271, and the 
highest of the ordinary stock 5651. 

It is therefore unsafe to use figures greater 
than these, when the stock is such as was 
tested. For convenience of reference, for 
those who are accustomed to use the constant 
in some other form than the modulus of 
rupture, I have given the constant in the 
form used by Trautwine in his handbook, 
and also that in the form used by Barlow, the 
former being the calculated central breaking 
weight of a beam one inch square and one foot 
long, this being one-eighteenth of the 
modulus of rupture, and the figures in the 
last column being one-sixth of the modulus 
of rupture. 

In the case of Nos. 14 and 15 we have two 
pieces of timber of rather small dimensions, 
which had been seasoning on the wharf for 
four' years, without being exposed to stress 
or strain. These gave very high moduli com- 
pared with the others, the first one being 
tested flatways, like a plank, and giving 8748, 
while the last, which was tested edgewise, 
gave 7562. 

Now, while my object is to ascertain the 
facts in these matters and to teach these facts 
to my students by giving them the oppor- 
tunity to ascertain them for themselves; 
nevertheless, it is a perfectly proper question 
to ask how these moduli of rupture compare 
with those comjnpnljy given by oiar text books 



and engineers' handbooks. To answer this 
question I will give below the values of the 
moduli of rupture of spruce given by difi'erent 
authorities, for the sake of comparison, de- 
ducing the modulus of rupture from their con- 
stant when this is given in some other form : 
Max. Min. Mean. 

Hatfield gives 12,996 7,506 9,900 

Rankine gives 12,300 9,900 11,100 

Laslett gives 9.707 7.506 9.045 

Trautwine gives 8,100 

Hodman gives 6,168 

Hatfield's, Laslett's, Trautwine's and Eod- 
man's figures are from their own experiments, 
and as is evident from the above table, they 
differ very considerably. Trautwine advises 
for practical use to deduct one-third, on 
account of knots and defects, hence after 
making this allowance he would use 5400. 
My own figures would give this only for the 
best of the stock in common use, and in the 
case of knotty and poor timbers would give 
much lower values ; I obtained as low as 3271 
and an average of 4623. 

Another question which it is perfectly fair 
to ask, and perfectly reasonable to expect me 
to answer is. Why these differences? The 
main reason seems to me to lie in the fact that 
the experimenters have generally used very 
small pieces, one or two inches square and 
about five feet long; now such pieces are 
taken free from the knots, and cracks, and 
defects, and also from the lack of homogene- 
ity> that are always found in larger timber. 
Besides this, there are the facts that the 
experimenters generally use straight-grained 
and well-seasoned timber, and also that tim- 
ber has a much better chance for seasoning 
when in such small pieces than when in large 
ones. The figures obtained by Eodman are 
very much smaller than the others, and it 
looks as though they must have been obtained 
from pieces somewhat larger than those 
usually experimented upon ; they are not at 
all as small, however, as those which I obtain. 
On this modulus of rupture (Eodman's) are 
based the tables given in the handbook of 
Messrs. Stetson & Moseley. 

We need of course to prosecute the subject 
further and to make an extensive series of 
experiments determining the strength of tim- 
ber obtained from various places and of vari- 
ous degrees of seasoning, and also of old tim- 
ber that has been in use, and to determine as 



8 



far as possible the effect of certain defects, 
such as the cracks that are constantly found 
at the heart of the timber, and what we have 
already done shows that these cracks have, at 
least in certain cases, a very decided effect in 
weakening the timbers. Also the effect of 
time on the breaking weight should be ex- 
amined. Of course we shall hereafter follow 
up these lines of investigation, not only in 
the case of spruce, but also in the case of 
other kinds of timber used in construction, 
such as yellow pine, etc. ; also the deflection 
and stiffness of timber used in construction. 
In this regard we have thus far tried three 
experiments, which gave for the modulus of 
elasticity of the timber used as follows : 

No. 3 1,237,215 

No. 4 1,067,893 

No. 6 938,453 

3) 3,243,561 

Average 1,081,187 

I do not pretend to say that this result 
would be a correct average value for the mod- 
ulus of elasticity of ordinary spruce timber, 
as the determination of that would require 
more than three experiments; but this can 
be used until we have more light on the sub- 
ject, and it will be found to be far below the 
value deduced from tests of small sticks such 
as are commonly experimented upon. 

At the close of the meeting the audience 
were invited to witness the breaking of a 
4x12 inch spruce beam centrally loaded 18 
feet between the supports. The beam was the 
best that could be selected of that size on the 
previous day from Mr. J. W. Leatherbee's pile 
of lumber, and was fully up to the average of 
the ordinary stock. It broke with a load of 
8000 pounds. This gives for modulus of rup- 
ture 4500 pounds per square inch. 

Since the above paper was presented to the 



society some other developments have oc- 
curred, which I will proceed to mention : 

1. Desiring to prove by actual comparative 
test the truth of the ideas I advanced in the 
paper, I proceeded as follows : About a month 
before the meeting I had bought at a carpen- 
ter's a very good, straight grained spruce 
plank which had been lying in a warm 
room for a long time, perhaps a year. I had 
him cut off for me some little beams about 
1.5x1.75 inches, and then tested the remainder 
of the plank, which was 2x6i inches, as a 
beam edgewise, with an eight foot span. It 
gave a modulus of rupture of 7160, and sub- 
sequently testing the small pieces, which had 
been lying in my study for about a month, 
we obtained as moduli of rupture respectively 
12,323, 13,350, 13,997, thus bearing out very 
neatly the ideas already advanced. 

2. My attention has been called by a friend 
to an article in Engineering, of April 23 and 
May 7, 1875, by Mr. C. Graham Smith, giving 
an account of some results obtained by him , 
by experimenting on a few large beams. 
While he gives no results for spruce, his 
moduli of rupture for other woods are much 
lower than those obtained by experimenters 
who have used small pieces. 

3. I have just received a letter from Col. T. 
T. S. Laidley, of the Watertown arsenal, in 
which he says: "I have just come across 
some of the old specimens used by Eodman in 
determining the transverse resistance of 
woods. The pieces are 5 feet in length, 5| 
inches deep, and 2i inches wide. The distance 
between supports was about 50 inches." This 
bears out very neatly my suspicions in regard 
to Rodman's experiments, as the pieces he 
used would naturally contain more defects 
than the very small pieces ; nevertheless, my 
tests on full size pieces give even smaller 
results in most cases. 



1 TRRARY OF CONGRESS 

HURU. 

019 418 217 ftj* 



■"» 



/ 



" 019 418 217 J 



